Inductive load circuit supplied with rectified alternating electric current



Oct. 21, 1941.

s. A. STEVENS 2,260,200 INDUCTIVE LOAD CIRCUIT SUPPLIED WITH RECTIFIED ALTERNATING ELECTRIC CURRENT Filed March 8, 1940 Patented Oct. 21 1941 UNITED STATES PATENT OFFICE INDUCTIVE 'LOAD' CIRCUIT SUPPLIED WITH RECTIFIED ALTEBNATING' ELECTRIC CURRENT Sydney Arthur Stevens, Kings Cross, London, England, assignor, by mesne assignments, to The Union Switch and Signal Company, Swiss.- valc, Pa., a corporation of Pennsylvania Application March 8, 1940, Serial No. 322,989 In vGreatBritain June 20, 1939 1 Claim.v (Cl. 175-363) This invention relates to inductive load circuits arranged to be supplied with uni-directional currents derived from alternating currents of the usual periodically varying wave form through rectifying apparatus of the dry surface contact a rectifier, the current input to the rectifier is. of the usual sinusoidal wave form, but the current. in the inductive circuit willonly vary Very.

slightly during a cycle due to the inductance of the circuit. As a result, relatively highvoltagepeaks will occur during each wave cycle; which;

impose corresponding severe stresses on the rectifier. T ese periodic excessive voltages-occurring, in'each cycle may be arranged to be prevented by a suitable designed condenser connected. in parallel with the load circuit,but when the. mean value of the current in the inductive loadv circuit is varied as, for instance when the motor, is started into operation or its load is suddenly.

increased, then even higher transient voltage stresses are set up which are in general indeterminate, being dependent, amongst other conditions, upon the point in the cycle at which the variation of the load circuit current is effected. In the case of an alternating current generator, similar sudden increases in current can arise when the load is suddenly thrown on the alternator, or when a fault occurs on the network supplied by the generator. These voltage stresses are liable to cause the break-down of the rectifier so as to destroy its resistance in the reverse direction so that the rectifier becomes inoperative and may even be destroyed.

According to the principal feature of the invention, a safety device is provided in the form of a relatively small dry surface contact rectifier device adapted to break down so as to become electrically conductive under an impressed voltage which is substantially less than that required to break down the main rectifier, the safety device being connected in parallel with the main rectifier and preferably so mounted as to be These clips are permanently connected: to the readily removable from the circuit. and when removed, tocause the main rectifier to be automatically short-circuited.

In: carrying the invention into practice, the safety. device preferably comprises a small dry. surface. contact rectifier unit consisting of, two

oppositely connected rectifier discsor sets'ofv discs mounted in a suitabletube or casing and provided. with contact terminals at each endv adapted to engage in suitably mounted spring clips in a similar manner to a cartridge type fuse;

terminals of the main rectifier and are-also provided with contact springs projecting towards one another so as to overlap and arranged to be maintained out of electricalengagement with one another by aprojection on the safety device when the latter is inserted into position in the spring clips.

In operation, so long as the voltage developed at the terminals of the main rectifier and. thus appliedxacross the spring clips and the safety device is not abnormal, the safety device is not.

traversed by any appreciable current owing, to

the opposing action of the rectifier elements ofwhich'the deviceis composed.

When however, the voltage developedat the terminals of the main rectifier and of the-device attains a predetermined value correspondingto the break-down voltage of the safety device, the:

latter becomes operative and permits the passage ofcurrent in both directions, thereby short circuiting and thus preventing injury to the main rectifier.

Under these conditions the safety device must be removed and replaced by a fresh device, the main rectifier being short-circuited by the action of the contact springs above described until this replacement has been effected.

In order that the present invention may be readily understoood, it will now be described by way of example with reference to the accompanying drawing in which,

Figure 1 is a circuit diagram of an arrangement embodying the invention, and

Figures 2 and 3 are somewhat diagrammatic views illustrating the construction of two devices suitable for use in connection with the invention.

Referring now first to Figure 1, the conductors of a circuit through which alternating current is supplied from a generator or to a synchronous motor are shown at I, a current transformer 2 having its primary winding 3 connected in series in one of the conductors I.

The terminals of the secondary winding 4 and of the transformer 2 are connected to the input terminals 5, B of a main bridge-connected dry surface contact rectifier device 1, the output terminals 8., 9 of which are connected to the terminals of a field winding ll] of the generator or motor (not shown).

Connected across the terminals of the secondary winding 4 and of the transformer 2 in parallel rectifier elements of the dry surface contact type;

A pair of spring contacts 12 are provided which are normally held apart from one another so long as the safety device H is in position in a clip device provided therefor, the contacts l2 being arranged to come into electrical contact with one another so as to short-circuit the main with the main rectifier device I is a safety device I H comprising as shown two oppositely connected rectifier device I and the secondary winding v l when the safety device H is removed.

The device ll is so designed as to withstand a only a Voltage which can be safely applied to the main rectifier device I, but in theevent of any higher voltage being developed at the terminals 5, 6 of the rectifier device I, the rectifier elements of the safety device H are broken down and the main rectifier 'l' is short-circuited and thereby prevented from being injured.

Under these conditions, the safety device ll may be removed for replacement and until this has been effected the main rectifier device 1 will remain short-circuited by the contacts l2 as circuiting the main rectifier device I, the input terminals 5, 6 of which are permanently connected to the terminals 0 of the contact blocks b and c.

The same result is achieved in a somewhat different manner in the construction illustrated in Figure 3, in which the movable contact is constituted by a resilient blade is on which a pin m .7 inductive load constituted by a field winding of an alternating current generator or synchronous motor, this winding being supplied with unidirectional current through a current transformer and a rectifier for the purpose of efiecting series compounding of the generator or motor.

The invention may however, also be applied to above explained, the field winding It being inopl erative for the time being.

Figures 2 and 3 illustrate in detail two arrangements for this purpose. In the arrangement illustrated in Figure 2, the safety device comprises a rectifier cartridge g similar in appearance to a cartridge fuse and clamped between a contact block b fixed to an insulating base a. and a movable contact block 0 which is resiliently supported on the base a by means of a leaf spring at and provided with a contact blade e adapted to co-operate with a fixed contact element 1, the latter being electrically connected to the fixed contact block b. When a rectifier cartridge 9 is mounted as shown between the contact blocks b and c, the contact blade 6 is held in a position in which it is out of engagement with the fixed contact element 1, but when the cartridge 9 is removed, the spring at will move the contact block 0 in an anti-clockwise direction, so as to permit the contact blade e moving into engagement with the contact element thus automatically shortcircuits embodying an inductive, load of other character and supplied with current in a similar manner for protecting the main rectifier from liability to injury by transient excessive voltage sot-s s Having now particularly described and ascertained the nature of my said invention and in what ,manner the same is to be performed, I declare that what I claim is:

A safety device for an inductive load circuit supplied from ,an alternating current source through a main rectifier, said device consisting of a pair of relatively small dry surface contact rectifier units connected in series in a direction to oppose one another across the input terminals of said main rectifier, said rectifier units being adapted to break down so as to become electrically conductive under an impressed voltage from said source which is substantially less than that required to break down said main rectifier, a mounting for said rectifier units which renders said safety device readily removable, and means effective when said safety device is removed for causing said main rectifier to be automatically short-circuited, for the purpose specified.

SYDNEY ARTHUR STEVENS. 

